1. Field of the Invention
The present invention relates to the field of integrated electronic circuit technology. More particularly, the invention relates to a reliable and manufacturable capacitorlike, electrically-programmable interconnect device to be used in integrated circuits.
2. The Prior Art
Integrated electronic circuits are usually made with all internal connections set during the manufacturing process. However, because of high development costs, long lead times, and high manufacturing tooling costs of such circuits, users often desire circuits which can be configured or programmed in the field. Such circuits are called programmable circuits and they usually contain programmable links. Programmable links are electrical interconnects which are either broken or created at selected electronic nodes by the user after the integrated device has been fabricated and packaged in order to activate or deactivate respectfully the selected electronic nodes.
Programmable links have been used extensively in programmable read only memory devices (PROMs). Probably the most common form of programmable link is a fusible link. When a user receives a PROM device from a manufacturer, it usually consists of an X-Y matrix or lattice of conductors or semiconductors. At each cross-over point of the lattice a conducting link, called a fusible link, connects a transistor or other electronic node to this lattice network. The PROM is programmed by blowing the fusible links to selected nodes and creating an open circuit. The combination of blown and unblown links represents a digital bit pattern of ones and zeros signifying data which the user wishes to store in the PROM.
Such fusible link PROM systems present certain disadvantages. For instance, because of the nature of the conducting material in the link, relatively high voltage and high current levels are needed during programming to guarantee the complete blowing of the fusible links. Since the link is usually conductive, it needs large amounts of power dissipation to blow it. Also, the shape and size of the fusible link must be precise so that the link will function effectively as a conductor if it is not blown and will be a completely open circuit if it is blown. Therefore, very critical photolithographic steps and controlled etch techniques are required during the manufacturing process of fusible link PROMS. Finally, a large gap must be blown in the link in order to prevent it from later becoming closed through the accumulation of the conducting material near the blown gap. Fusible link memory cells are relatively large in order to accommodate the link and its associated selection transistor and, therefore, fusbile link PROM systems have high manufacturing and material costs and take up large amounts of chip real estate space.
In recent years, a second type of programmable links, called anti-fuse links, have been developed for use in integrated circuit applications. Instead of the programming mechanism causing an open circuit as is the case with fusible links, the programming mechanism in anti-fuse circuits creates a short circuit or relatively low resistance link. Anti-fuse links consist of two conductor and/or semiconductor materials having some kind of a dielectric or insulating material between them. During programming, the dielectric at selected points in between the conductive materials is broken down by predetermined applied voltages, thereby electrically connecting the conducting or semiconducting materials together.
Various materials have been suggested for the dielectric or insulating layer. Some of these suggested dielectric materials require a relatively high current and voltage during programming, require complex manufacturing techniques and have low reliability during programming because it is difficult to control the reproducibility of the conductive state due to the nature of the crystalline structures of the materials involved. In addition, the programming process results in a link having a finite resistance in the order of several hundred to several thousand ohms. This characteristic of the known anti-fuse elements renders them relatively unsuitable for use in high speed circuits.
Some of the proposed dielectric insulators are doped amorphous silicon alloys, polycrystalline resistors, oxides, titanate of a transition metal, silicon oxide, aluminum oxide and cadmium sulfide. The problems with these approaches, have been related to the need of a high current and voltage to program and the difficulty to manufacture and control their reliability in both the on and off states. Materials such as cadmium sulfide, aluminum oxide and titanate, present complicated technological problems because they are difficult to integrate into standard semiconductor processing. Capacitors with silicon oxides used as a dielectric do not produce a low enough impedance after programming.
Examples of known anti-fuse elements are found in the prior art using various insulating materials. Reference is made to: U.S. Pat. No. 3,423,646 which uses aluminum oxide, cadmium sulfide; U.S. Pat. No. 3,634,929 which uses single film of AL.sub.2 O.sub.3, SiO.sub.2, and Si.sub.3 N.sub.4 ; U.S. Pat. No. 4,322,822 which uses SiO.sub.2 ; U.S. Pat. No. 4,488,262 which uses oxide or titanate of a transition metal; U.S. Pat. No. 4,499,557 which uses doped amorphous silicon alloy; U.S. Pat. No. 4,502,208 which uses SiO.sub.2 ; U.S. Pat. No. 4,507,757 which uses SiO.sub.2 ; U.S. Pat. No. 4,543,594 which uses SiO.sub.2.
Most of the above patents either describe complicated technologies or need high breakdown voltages and currents, and or are difficult to manufacture or do not meet the reliability requirements of state-of-the-art integrated circuits in both the on and off states. These patents do not disclose the creation of controllable conductive filaments with low resistance after programming.
Other problems associated with existing dielectric materials in anti-fuse links include large memory cells, and complex manufacturing processes for the unblown anti-fuse elements.